1. Field of the Invention
This invention relates to methods, means, and systems for providing gain control within a media read channel.
2. Description of the Related Art
Magnetic media such as magnetic tape is typically encoded at regular intervals with a calibration region that facilitates synchronization and gain control within a read channel. One common method is to encode a sinusoidal waveform having a normalized frequency and amplitude with a Variable Frequency Oscillator (VFO). Regions on the media containing the normalized waveform are known as VFO regions.
FIG. 1 is a block diagram depicting selected portions of one embodiment of a currently available read channel 100. As depicted, the read channel 100 includes a sampling module 110, a VFO (region) detector 120, a stream synchronization module 130, a gain control module 140, a mixer 150, and a channel decoder 160. Multiple read channels 100 may be incorporated with a multi-track recording device such as a tape drive (i.e. one channel for each track).
The sampling module 110 samples the magnetic media in conjunction with a read head (not shown) or the like and provides an asynchronous digital stream to the VFO detector 120 and the stream synchronization module 130. The stream synchronization module 120 may normally operate in a tracking mode and include a phase-locked loop to track variations in the data rate of the digital stream. In response to the VFO detector detecting a calibration signal, the stream synchronization module may change to an acquisition mode in order to precisely calibrate (i.e. lock) the clock rate and signal gain of the read channel 100.
Subsequent to clock synchronization, the gain control module 140 may begin a gain normalization process by detecting the signal level of the synchronized (i.e. synchronous) digital stream. To achieve the required precision, a number of cycles of the VFO waveform may need to be processed.
After calibration, the read channel may resume processing in a tracking mode and provide an amplitude-normalized synchronous sample stream to the channel decoder 160. In certain systems, the channel decoder 160 is essentially a PRML estimator that processes a slightly oversampled synchronous sample stream. To be effective such an estimator requires that the sample stream be precisely normalized and tightly synchronized.
Applicant has observed that the precise and sequential nature of the calibration process in currently available high data rate read channels results in calibration regions that are quite lengthy. Furthermore, in multi-track systems, tape skew may further increase the required length of the VFO or similar calibration region resulting in large portions of the media being unavailable for data storage.
From the foregoing discussion, it should be apparent that a need exists for an apparatus, system, and method for gain control calibration that operates concurrently with clock synchronization. Beneficially, such an apparatus, system, and method would reduce the duration and media real estate necessary to precisely calibrate a read channel.